I. Field of the Invention
The present invention relates generally to a method and apparatus for minimizing electromagnetic interference (EMI) from a power inverter of an electric vehicle.
II. Description of Related Art
Electrically powered automotive vehicles, and especially hybrid automotive vehicles, are becoming increasingly popular due to the fuel economy achieved by such vehicles.
All such electric vehicles utilize a power inverter to power the electric motor for the vehicle. These power inverters must be capable of producing relatively high power outputs, typically 50 horsepower or more. As such, the power inverters necessarily must be capable of switching high current loads.
A major source of EMI radiation from the power inverter arises from the ground bouncing of the control circuit and its base plate for the power inverter. Conventionally, the circuit board, which has its own ground plane, is mounted to a metallic base plate by first fasteners. Second fasteners are then used to secure the base plate to the power inverter case.
Ideally, the ground plane for the control circuit will be identical to the ground plane for the inverter case. In practice, however, relatively wide spacing occurs between the first and second fasteners. This relatively wide spacing gives rise to parasitic inductance between the ground plane of the control circuit and the case. At the high switching frequencies utilized by power inverters, these parasitic inductances result in the generation of EMI. When the drivers/switches of hybrid control circuit drive the electric motors or loads outside inverter, all return currents flow through the inverter case and the base plate of the board. In addition, the common mode return current from motor PWM switching also flow through the same path as other return current. Then, the parasitic inductance of the return path causes the voltage bouncing on the ground of the motor control board due to the return current flow. The voltage bouncing of ground creates significant noises for radiated and conducted emission from inverter system.
Also many bus bars are deployed to carry very high currents and become the source of magnetic field inside the power inverter enclosure, which couple with other signal harness to generate additional conducted emissions. All these noises accumulate and male it more difficult to pass OEM EMC/EMI requirement in component/vehicle level EMC test. Therefore, the effective EMC noise attenuation method is important to manufacture high quality power inverter module for HEV application.
For safety reasons, the power inverter is typically housed in a metal case having one or more electrical ports, each of which is adapted to be connected to an external electrical load by a cable connector. Such power inverters also include a control circuit mounted to a circuit board which is also contained within the interior of the case for the power inverter.
One disadvantage of the previously known electric vehicles, however, is that the high voltage and high amperage switching within the interior of the power inverter generates extensive electro-magnetic fields as well as electromagnetic interference (EMI). Such EMI can cause interference to components of the vehicle, such as radio interference, and in extreme cases may interfere with the operation of the vehicle itself.
Another source of EMI in the power inverter results from the operation of the high voltage interlock (HVIL) system of the power inverter. In the conventional HVIL system, an electrical sensing wire extends serially to each electrical port on the power inverter case. When an electrical connector is properly attached to its associated electrical port, the electrical connector completes the connection for the HVIL sensing wire thus maintaining continuity of the circuit. The HVIL sensing wire is coupled to a circuit which monitors the electrical continuity of the HVIL sensing wire.
Consequently, in operation, in the event that a cable connector becomes disconnected from its associated electrical port thus posing a potential safety hazard, the electrical continuity of the HVIL sensing wire is interrupted. This interruption in turn is detected by the circuit which then takes the appropriate action, e.g. shutting down the high voltage and high amperage circuitry of the power inverter.
While the operation of the previously known power inverters with an HVIL system provides adequate safety for the power inverter by detecting a disconnected cable connector, the high magnetic switching fields within the interior of the power inverter case in turn induce high frequency signals into the sensing wire for the HVIL system. The HVIL sensing wire in turn generates extensive EMI which is generated exteriorly of the power inverter case since a portion of the HVIL sensing wire extends through each cable connector and thus exteriorly of the inverter case.